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Energies 2019, 12, 457 15 of 35 two-stage CO2 refrigeration systems, Liu et al. [83] found that the gas cooler outlet temperature and the low-stage compressor significantly affect the system performance. Recently, the BoostHEAT Company introduced a new and original concept of a thermal compressor for transcritical CO2 HP. Unlike the rotary or scroll compressors where the mechanical power generates the pressure increase of working fluid, in the new design, a heater provides thermal energy to warm up the working fluid of the compressor and then to increase its pressure. The uniqueness of the system is that the same working fluid can be used in both the thermal engine and the HP to enhance thermal efficiency. To analyze the thermal performance of the compressor, Ibsaine et al. [84] formulated a mathematical model and recommended that a two-stage thermal compressor is preferred in CO2 HP applications. 3.4. Expansion Device The prime functions of the expansion device in a CO2 HP are to distribute CO2 to the evaporator as well as to maintain a pressure difference between the evaporator and the gas cooler. Most of the recent investigations focused on different kinds of expansion device designs (e.g., ejectors, expansion valves) and their configurations in terms of the numerical modeling, geometrical structure, and system performance enhancement. Theoretical and experimental investigations suggest that an ejector in place of an expansion valve can improve transcritical cycle performance [85–87]. The refrigerant inside the ejector is characterized by two-phase and compressible flow. A homogeneous equilibrium model (HEM) and homogeneous relaxation model (HRM) are the two widely used numerical models for the ejector flow [88]. HEM considers liquid and vapor phases are in a homogeneous equilibrium whereas HRM uses the empirical correlation of the relaxation time. Lucas et al. [89] and Palacz et al. [90] employed HEM to predict the two-phase flow inside an ejector. The HEM accuracy decreases with a decline in a motive nozzle temperature and increases in deviation from the saturation line, which demands a more complex HRM mode. Angielczyk et al. [91] studied HRM for a supersonic flow through the convergent-divergent nozzle of a CO2 ejector and validated their model using three different nozzle diameters. They developed a widely excepted correlation for relaxation time considering the critical flow parameters, vapor quality, and temperature profile. Through a comparison study, Palacz et al. [92] found that HRM could predict the flow characteristics better than HEM (overall a 5% increase in accuracy). In addition, Zheng et al. [93] and He et al. [94] proposed dynamic numerical models, which are useful in predicting the dynamic system response in different operating conditions to optimize the ejector expansion performance. In a CO2 transcritical cycle, expansion or throttling losses result in a low system performance. Such throttling losses can be overcome by adopting multi-ejector systems (connected in parallel or in a different combination). A study on a two-ejector system has shown that throttling losses could be substantially reduced, and the heating COP could be increased by about 10% to 30% compared to the basic two-stage cycle with a flash tank [29]. Furthermore, Boccardi et al. [30] experimentally studied the benefit of integrating a multi-ejector in a transcritical CO2 cycle for heating applications. They found that the optimal multi-ejector configuration could reduce the throttling losses by 46%, and, thereby, improve the system performance by up to 30%. Bai et al. [95] utilized two cascaded ejectors in a dual temperature transcritical CO2 system, which outperformed the basic system without the ejector. Additionally, the exergy efficiency and COP of the cascaded ejector system increased up to 25% and 28%, respectively, when compared to a single ejector system due to a higher pressure lift and better pre-compression. In another study, Bodys et al. [96] introduced swirl motions at the inlet of the motive and suction nozzle of fixed ejectors of a CO2 multi-ejector system. Swirl motion enhances the momentum exchange in the mixing section of the ejector and, thus, increases the contact time between the primary and secondary flow stream. They found that the system performed better due to the swirl motion at the inlet of the motive nozzle, but the swirl motion at the suction nozzle did not have any impact on the performance. Figure 4 shows the variation in ejector efficiency with swirl RPM.PDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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